Page 60 - Introduction to Transfer Phenomena in PEM Fuel Cells

Introduction to Hydrogen Technology 49
1.3.4.1. The supply system
The flow and pressure of the gas circuits is controlled. These control
systems make it possible to control the supply conditions of the fuel cell.
To improve performance, it is possible to work at high airflows in order
to evacuate the water produced and to limit the effects of the low partial
pressure of oxygen in the air. However, the use of compressors confers a
high energy cost [RAM 05].

At the anode, there is little increase in the hydrogen stoichiometry and it
is necessary to integrate a hydrogen recirculator to recover the unused
hydrogen (output) and reinject it (input). The oxygen supplying the fuel cell
is usually taken from the ambient air to prevent the storage of oxygen.
However, the air only contains 21% of oxygen, and in order to increase the
partial pressure of oxygen, it is sometimes necessary, for high power, to
compress the air. The use of a compressor is also to the detriment of the
overall efficiency of the system. It is powered by the battery and uses
significant energy, accounting for approximately 20% of the electric power
delivered by the battery [BLU 07]. Finally, there is the storage device (or
production) of hydrogen. Tolerance of combustible gases to impurities is an
important parameter. Some poisons such as sulfur or carbon monoxide can
seriously damage the battery. In the case of prototype cells, the hydrogen
used is usually a very pure gas [RAM 05].

1.3.4.2. The control system
As seen previously, the system has a large number of interconnected and
interdependent ancillaries, in order to ensure the proper functioning of the
system (overall efficiency, operational safety, taking into account
constraints).

It is necessary to have an overall control system to control the different
subsystems (static converter, gas flow, compressor control, water pump
required for cooling, humidifier temperature, etc.).

1.3.4.3. The static converter
The static converter makes it possible to form the electrical energy
produced by the fuel cell. The voltage of the battery, which is relatively low,
is not constant as a function of the delivered current. The converters are of
the following types: continuous DC, voltage booster (low voltage, high